1. Field of the Invention
The invention relates to the water rinsing of metal articles, such as containers to remove therefrom oil residues retained from metal shaping operations and more particularly to the use of ultrafiltration or reverse osmosis membranes in the preparation of the rinse water used in the article rinsing operation.
2. The Prior Art
A large proportion of metal cans presently used for the packaging of carbonated beverages are fabricated in two-piece form wherein the can body and one can end are integral and a separate end is later attached to the open end of the body to seal the container.
One method by which two-piece cans are manufactured is the so-called drawn and wall iron process wherein a shell cup is produced from sheet metal by drawing and is then redrawn to a cup of longer length and smaller diameter and the redrawn cup is then wall ironed to produce the required body length and wall thickness.
The container body after wall-ironing contains trace amounts of the oil used to lubricate and cool the shaping dies during the wall ironing operation.
The coolant is generally an oil-in-water emulsion comprised primarily of about 1 to about 20% by weight of a lubricant oil and about 0.1 to about 20% by weight of an emulsifying agent or surfactant which promotes the emulsification of the oil and water diluent to an emulsion of high stability; i.e., the oil cannot be separated from the water diluent.
The oil residue adhered to the container body must be removed in order that the protective coatings and enamels subsequently applied to the container bodies will adhere properly. To remove oil and other foreign material which adheres to the drawn container, the container body is rinsed with water sprays. After the water rinsing the container is further washed and chemically treated to prepare the container for the coating and decorating operations which follow.
The rinse water, after it is used to effect the removal of the oil residue from the container body, represents a waste disposal problem since the oil and other materials suspended in this rinse water are undesirable in drain lines and are either unlawful under community sewage codes or cause stream pollution or other disposal problems.
Among the methods known to the art for removing and recovering organic resinous materials suspended in water is ultrafiltration and reverse osmosis. In ultrafiltration or reverse osmosis a water suspension containing organic resin solids is fed under pressure to the "upstream" or feed zone of an ultrafiltration or reverse osmosis unit equipped with a semipermeable membrane mounted on a porous support housed in a pressure resistant structure and adapted to separate high molecular weight organic compounds from aqueous media. The water portion of the suspension passes through the semipermeable membrane and the organic resin is retained whereby a concentrated suspension of organic resin solids is formed in the feed zone side of the membrane. This concentrated solids suspension or concentrate is then removed from the feed zone and cycled for reuse. The water separated from the suspension, referred to as "permeate" having the solid organic resin removed therefrom can be readily disposed of without ecological problems.
The removal of organic resin solids by ultrafiltration or reverse osmosis from the aqueous effluent of coating processes is well known to the art. For example, U.S. Pat. Nos. 3,538,901 and 3,749,657 teach a method of removing organic resin solids from electrodeposition rinse water wherein the rinse water is subjected to ultrafiltration or reverse osmosis processes. The concentrated resin suspension obtained by ultrafiltration or reverse osmosis is returned to the electrodeposition bath for reuse and the permeate may be used as rinse water.
One drawback to using ultrafiltration or reverse osmosis processes for removal of organic solids from the rinse water used in the washing of drawn and wall ironed containers is that the oil which is separated from the effluent is not reusable in the can fabricating process which not only increases the cost of the process but presents a disposal problem.